A super absorbent polymer (SAP) is a type of synthetic polymeric material capable of absorbing 500 to 1000 times its own weight of moisture. Various manufacturers have denominated it with different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), and the like. Such super absorbent polymers started to be practically applied in sanitary products, and they are now being widely used not only for hygiene products such as disposable diapers for children, sanitary napkins, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, or the like.
In most cases, the super absorbent polymer is widely used in the field of hygiene products such as diapers and sanitary napkins, and, for this purpose, it is necessary to exhibit a high absorption capacity for moisture and the like. In addition, it is necessary that the absorbed moisture should not leak out even under external pressure. Further, it needs to show excellent permeability by maintaining its shape even in an expanded (swelled) state after absorbing water.
However, it has been known that it is difficult to improve both the water retention capacity (CRC) indicating basic absorption capacity and water retention ability of the super absorbent polymer, and the absorption ability under pressure (AUP) indicating the characteristic of holding the absorbed moisture even under external pressure together. This is because, when the overall cross-link density of the super absorbent polymer is controlled to be low, the CRC may relatively increase, but the AUP may decrease because the cross-linked structure becomes loose and the gel strength is reduced. On the other hand, when the cross-linking density is controlled to be high for improving the AUP, the basic CRC may decrease because moisture is hardly absorbed through the dense cross-linked structure.
For this reason, there has been a limit to provide a super absorbent polymer improved in both water retention capacity and the absorption ability under pressure. In order to solve this problem, there have been various attempts to improve these properties by controlling the type or amount of the internal cross-linking agent or the surface cross-linking agent. However, such attempts have reached the limited.
Therefore, there is a continuing need to develop a super absorbent polymer exhibiting improved water retention capacity and absorption ability under pressure together, and a technology capable of producing the same.